The present invention relates generally to systems and methods for etching digital watermarks and, more particularly, to systems and methods for etching digital watermarks with improved error tolerances.
The secure transmission of multimedia information is an important concern for multimedia service providers and multimedia content owners. Secure conditional access has conventionally been accomplished through the use of encryption. However, for widespread distribution of multimedia, conventional key-management mechanisms used in encryption techniques are often too cumbersome. In addition, encryption techniques do not address the issue of copyright protection. Once encrypted material has been decoded, the reproduction, manipulation, and redistribution of data streams can be almost effortless, especially since information transmitted in packet networks is in a digital format.
More recently, attention has been focused on supplementing encryption with digital watermarking to provide copyright protection and tamper-proofing for multimedia streams. Digital watermarking is a technique that hides information, or a message authentication code (MAC), in the audio, image, or video stream. Watermarking has advantages over traditional MAC systems in that watermarks are embedded directly into the multimedia information (e.g., into the coefficients of the image or video signal or into the tones of the audio signal). A traditional digital MAC can be removed easily because it is attached to the document and jointly transmitted or stored.
In addition, conventional cryptographic techniques can be inadequate if the communication channel is lossy. A single bit error can cause the received signal to fail authentication when conventional MACs are used. If a robust MAC scheme such as watermarking is used, minor distortions will be ignored so that decisions based on the watermark can be made more intelligently and can more accurately reflect the state of the content that the watermark is protecting.
A considerable amount of research has been performed to investigate robust watermarks that identify the owner and/or the purchaser of the audio track or image in question. The primary requirement for copyright protection watermarks is that they be survivable to compression and common signal distortions, thus, making it impossible to remove the watermark without destroying the value of the multimedia content that it protects.
In addition to copyright applications, watermarking can also be used to authenticate and tamper-proof multimedia content. For a video authentication application, a watermark must be robust to compression, common signal distortions, and the effects of error-prone channels, but it must further indicate any modification made to the stream. Therefore, the watermark should describe the content and the salient features of the video information. For example, video authentication MACs can conventionally be constructed from edge maps of video frames or from an image histogram of video frames.
Digital watermarks are added to compressed video (e.g., MPEG video) by modifying the encoded video information. If a watermark is meant to be extracted without the use of the original, as is the case in authentication applications, the etching process must use some video coefficients as a reference.
One conventional method embeds a watermark by sliding a non-overlapping 3xc3x971 window over DC coefficients from luminance (Y) blocks in intraframes (I-frames). The coefficients contained in the window may be denoted b1, b2, and b3 to represent the coefficient values at the (ixe2x88x921)th, (i)th and (i+1)th position of the DC sequence. The corresponding rank-ordered coefficients are denoted as b(1)xe2x89xa6b(2)xe2x89xa6b(3). The median coefficient is then modified depending on the values of b(1), b(3), and the watermark bit x to be etched. The interval between b(1) and b(3) is divided into levels 1_0, 1_1, . . . , 1_m where b(1) is defined to be at level zero 1_0 and b(3) is defined to be at level m 1_m. b(2) lies at an arbitrary level 1_k between b(1) and b(3). If k is even and x=0, b(2) remains unchanged, but if x=1, then b(2) is increased (decreased) by one level making it fall on an odd level. Likewise, if k is odd and x=1, b(2) remains unchanged, but if x=0, then b(2) is increased (decreased) by one level making it fall on an even level. This transformation forces the coefficient b(2) to fall on an even level if x=0 and on an odd level if x=1. The coefficients b(1) and b(3) remain unchanged so that the original image is not needed during the extraction process.
Another conventional method uses an approach similar to that just described, except it uses only the first coefficient of each I-frame macro-block as a reference. The first coefficient is defined at level zero, and a watermark bit is etched into the second coefficient depending on whether the second coefficient lies on an even or odd level relative to the first coefficient. The same rules used above to determine if the median coefficient should be modified are applied to the second coefficient. The method then continues for the third coefficient using the second as the new reference, and then for the fourth coefficient using the third as the new reference, and so on.
One of the main difficulties in constructing a watermarking system for copyright protection or authentication applications is its survivability to acceptable signal distortions. For a copyright protection application, the list of acceptable video distortions is quite extensive because the content owner is interested in claiming ownership regardless of the manipulation done to the original content. For example, watermarking for copyright protection may need to be robust to cropping, rotating, dithering, recompression, and the like. The acceptable distortions for the authentication application are more limited because the authentication MAC must still be able to identify tampering. Distortions caused by recompression or by minor channel errors do not indicate tampering, while cropping and rotating do. During recompression, the essence of the video information is maintained, but minor fluctuations in individual coefficient values, though not perceptible, can change the value of the MAC. Robustness to distortion can be built into the watermark etching algorithm or into the MAC itself.
Therefore, there exists a need for a system and method that improves the survivability of an etched watermark to signal distortions induced in multimedia data.
Systems and methods consistent with the present invention address this need by improving the error tolerance of watermark etching algorithms. The present invention builds in an error tolerance buffer when watermark bits are etched into digital data. Using systems and methods consistent with the present invention, minor modifications (e.g., distortions) to digital data values that fall within a specified error tolerance range will not cause an error in the watermark extraction process. Modifications to digital data values above the specified error tolerance range (whether due to excessive recompression or to actual tampering) may alter the extracted watermark bits at the receiver and may indicate tampering.
In accordance with the purpose of the invention as embodied and broadly described herein, a method of watermarking digital data includes obtaining a matrix of digital data values; selecting a plurality of data values from the matrix; specifying a parameter that indicates an acceptable amount of variation in the data values; and etching watermark bits in at least two of the plurality of data values using the specified parameter.
In another implementation consistent with the present invention, a method of extracting a watermark from digital data includes obtaining a matrix of digital data values containing an etched watermark; selecting a plurality of data values from the matrix; specifying a parameter that indicates an acceptable amount of variation in the data values; and extracting watermark bits from at least two of the plurality of data values using the specified parameter.